The present invention relates to a process for preparing a coal-water slurry which is nearly the same, to handle, store and burn, as fuel oils, in particular a process for the preparation of a coal-water slurry having low ash content and high solid content with a high coal recovery as well as economically, regardless of the amount of ash content of parent coal.
It is well known to prepare a high solid content coal-water slurry by mixing a ground coal with water or wet grinding a mixture of crushed coal and water. In these preparations it is desirable to reduce the amount of ash which is mingled in the slurry accompanied by the coal as much as possible. The coal contains more or less of ash comprising Al.sub.2 O.sub.3, SiO.sub.2, FeS and the like. Mingling of ash in the coal-water slurry is disadvantageous in that when said slurry is burnt, not only is abrasion of the boiler walls brought about, but also the heating value of said slurry is lowered.
Under these circumstances, in the preparation of a high concentration coal-water slurry there has hitherto been employed a process which comprises subjecting a relatively coarse grain-sized parent coal to gravity classification to thereby obtain a low ash coal whose ash amount is in the allowable range, and grinding said low ash coal alone to obtain a coal-water slurry, or a process which comprises grinding the whole amount of a parent coal itself and thereafter subjecting it to deash treatment for obtaining a low ash coal. However, the former process is defective in that a certain amount of combustible matter is not transferred to said low ash coal and is not utilized as the combustible component for slurry, and therefore the coal recovery is low. On the other hand, the latter process is high in coal recovery as compared with the former process, but is economically unprofitable in that because a large amount of coal is subjected to deash treatment, there must be used a large-sized deash equipment and accordingly the costs of equipment and working the apparatus are high.